1. Field of the Invention
The present invention relates to a picture digitizing system which is effective when it is desired to reproduce a picture having a half-tone level with use of, for example, a dot printer or the like.
2. Description of the Related Art
Such a picture having a half-tone level as a photograph picture has been recently output and displayed with use of such a simple binary recorder as a dot printer. Since this sort of recorder cannot provide multiple display states more than two display states, white and black, it becomes necessary to effectively convert the above half-tone picture into binary one. With regard to the binary processing, for example, for photographs on newspapers, various screens having different net sizes are used to provide a varying density effect to the photograph. However, the employment of this method to the dot printer or the like as it is involves difficulties.
To avoid these difficulties, it has been common practice to typically introduce such an organization dither method or a mean error least method to convert a half-tone picture into binary one. In the organization dither method, a suitable threshold level is previously set for pixel positions and a pixel signal is discriminated and converted into binary one on the basis of the threshold level. However, this organization dither method is inferior to the above mean error least method in picture quality. In the mean error least method, on the other hand, binary processing (conversion to a binary data) is determined so that an average of differences between an original picture and binary pictures each made of a plurality of picture elements (pixels) in small zones of the original picture becomes small. This mean error least method however has a defect that, though the method can provide improved picture quality, the binary processing is comples and thus requires a large-scale system arrangement.
In view of such circumstances, there has been proposed such a picture digitizing system as taught in Japanese Patent Appln. Laid-Open No. 57-104369 wherein an average density approximation method for converting a half-tone picture to a binary picture is used to obtain the binary picture of a picture quality comparable to that obtained by the mean error least method, and also which can be inexpensively realized with a simple hardware arrangement. In this system, the binary conversion is carried out on the basis of an area ratio of black dots occupied in a small zone, i.e., an average density. The average density is calculated with respect to a dot to be converted to a binary dot as well as already-converted binary dots arranged around the former dot, and the decision of whether the dot to be converted is black or white is made by computing an average density of the dot to be converted when it is black and an average density thereof when it is white, checking which is closer to that of the input signal and employing the closer one.
However, the above picture digitizing system has had a problem that, when the system reads or inputs such a picture having a regularly varying density or a constant density as artificially created, a regular read level continues and therefore its output dot pattern becomes similar, which results in that the picture quality is deteriorated.
In addition, the prior art system has been disadvantageous in that, when picture reproduction is carried out through the conversion of density, .gamma. characteristics and so on of the picture, the binary conversion and then the picture processing must be performed, thus requiring a large size of hardware.